Natural abrasives performance characteristics and applications

Natural abrasives performance characteristics and applications

2021-11-09 13:00:16 8

Abrasive is a granular material with high hardness and certain mechanical strength, which is used for manufacturing abrasives or directly for grinding and polishing. As abrasives must have the following five conditions: high hardness, moderate resistance to breakage and self-sharpening, good thermal stability, certain chemical stability, and easy processing into particles of different sizes. Abrasives are divided into two categories: natural abrasives and artificial abrasives. All materials in nature that can be used for grinding or abrasive are collectively called natural abrasives. Good natural abrasives should not only be hard, but also tough and sharp, rich in resources or widely distributed, and also have a certain degree of purity. Although the preparation of man-made abrasives is becoming more and more refined, natural abrasives still have a certain status at present.

1、Natural diamond


Diamond (C) is the hardest known material, Mohs hardness is 10. common diamond crystal structure belongs to the equiaxial crystal system, vertical face center structure, typical of atomic crystal. Diamond also has hexagonal crystal system 2H polymorphic phenomenon that is hexagonal diamond, formed when the meteorite hit the surface of the earth, also found tetrahedral crystal shape of diamond.


Natural diamond is mostly 8-sided, and rhombic 12-sided, less cubic. There are obvious growth layers on the {111} face of natural diamonds, and the thickness of the growth layers is less than 10A, while no such growth layers are produced on the {110} and {100} faces. Most of the natural diamond is single crystal output, in round grain or broken grain shape. Pure diamond is colorless, but relatively rare, mostly in different colors, such as yellow, green, brown, rose, blue, gray, black, etc. Gem-grade diamond, i.e. diamond, is limited to colorless, nearly colorless, light yellow, pink, blue, green, etc. Other than that, other colors of diamond can only be used in industry. Diamond density is 3.50--3.52g/cm3, good thermal conductivity. The hardness of natural diamond varies according to its origin, deposit type and color. Diamond is brittle and has a parallel {11} medium solution, i.e., it is easy to crack along the {111} crystalline surface in the crystal under the action of external force. Diamond brittleness is also related to the crystal's internal stress, cracks and other defects. The greater the internal stress of the crystal, with cracks and other defects are also more, in the lower impact can be split.


The formation of natural diamond is divided into two major mineral systems, namely primary ore and alluvial ore. Primary ore is mainly kimberlite type primary ore, few deposits, bad quality, mostly for industrial use. Alluvial ores are the most important deposits for diamond production, and gem-quality diamonds are mainly produced from impacted alluvial ores. The world's most famous diamond sources are the Kimberley region of South Africa, Zaire, western Australia, Yakutia in Russia, Alaska in the United States and Sinasgiras in Brazil. In China, there are diamond mines in Liaoning, Shandong, Hunan and Guizhou, mainly primary ores, but the reserves are not large.


There are two main uses of natural diamonds as abrasives, namely dressing wheels, grinding and grinding difficult to machine materials (such as carbide, gemstones, glass, stone, etc.). The main tools are natural diamond wheel cutters, natural diamond dressers, natural diamond diamond pens, natural diamond bits, natural diamond turning tools, diamond files and glass processing tools. Different uses of diamond have different technical requirements, such as natural diamond for glass processing tools require crystal integrity, shape of 12-sided, 8-sided and transitional crystals; color is not limited; transparent or translucent; does not promise to have cracks, crystal internal promise to have tiny inclusions.


2、Natural corundum


Among the natural minerals, corundum is second only to diamond in hardness. Corundum (Al2O3) is a tripartite crystal system, the crystal has the nature of ionic bond to covalent bond transition, the structure is more compact. Single crystals are usually girdle-shaped, columnar, and aggregates are granular or dense blocks. It is generally blue-gray, yellow-gray, and black if it contains iron. Glassy luster, Mohs hardness 9, no decomposition. Often {0001} or {101} cleavage is produced by poly-sheet bicrystals. Density is 3.95-4.10g/cm3, chemically stable. Corundum containing chromium and red is called ruby, while those containing titanium and blue are called sapphire.


Natural corundum ore is called corundum sand (also called natural adamantine) when it contains impurities such as magnetite, hematite, quartz, etc., and has a granular aggregate of iron ore-like appearance. Corundum sand generally contains about 60% corundum, mostly greenish gray and black, density 2.7--4.3g/cm3, Mohs hardness 7--9. Corundum can be formed in the process of magmatism, contact metamorphism and regional metamorphism. Various genesis of corundum-bearing deposits or rocks, when subjected to weathering, corundum is often transferred into sand ore. The main production areas in China are Liuhe in Jiangsu and Yingshan in Hubei. This mineral raw material is abundant in many parts of the world. As abrasives, corundum sand is often used to make grinding wheels, oil stones, sandpaper, sand cloth or micro powder, abrasive paste, etc.


3、Garnet


Garnet refers to a class of silicate minerals whose chemical composition has the general formula A3B2[SiO4]. Which A represents the divalent cations Mg 2+, Fe 2+, Mn 2+, Ca 2+, etc., B represents the trivalent cations A1 3+, Fe 3+, Cr 3+, V 3+, etc.. The common ones are iron-aluminum garnet, magnesium-aluminum garnet, manganese-aluminum garnet, etc. Garnet has a dense structure and is often in perfect crystalline form, such as rhombic dodecahedron, tetrahedral trioctahedron or both aggregates. The aggregates are often dense granular or dense massive, with Mohs hardness of 6.5-7.5, density of 3.5-4.2g/cm3, and no decomposition. There are many types of garnet minerals, almost all of them can be used as abrasives, the commonly used one is iron-aluminum garnet.


As a natural abrasive, garnet is used to some extent in many countries due to its low industrial sales price and the fact that it has some qualities that people demand when it comes to certain specific applications. Garnet abrasives are granular, with sub-shell or jagged fractures, moderate hardness, good toughness, sharp edges, good self-sharpening, and can form angles and edges in constant crushing and grading, making its abrasive ability better than other abrasives. After garnet is treated by high temperature forging, its toughness increases and its grinding effect is very good, especially in the grinding and processing of wood, the durability is also higher than that of alumina abrasives.


In resin abrasives, garnet can be used as filler to replace 20%-30% of brown corundum, and the effect is ideal. Garnet is also an ideal environmentally friendly sandblasting material due to the absence of free silica. Garnet can also be made into solidified grinding wheels for grinding lenses and lenses, mainly resin bonded abrasives. With garnet abrasives as cutting media, waterjet cutting relying on high pressure hydraulic jets is applied to cut oil (natural gas) pipelines, steel and other components, which is a new, environmentally friendly and safe way of cutting. Garnet can get good results on printing plates, glass, ceramic products, leather, stone and so on.


The garnet mine area in Mengchuan County, Aba Prefecture, Sichuan Province is the most important garnet mining area in China, and the garnet here belongs to iron-aluminum garnet. Due to its low hardness and low melting point (1000-1200℃ for the garnet family), it is not suitable for grinding materials with high hardness (such as ferrous metals), nor is it suitable for type sand, refractory materials and sintered abrasives. However, when grinding materials such as silicon wafers and optical glass, it rarely produces abrasions and scratches, and this unique and excellent property is incomparable to artificial abrasives of the same specification. The garnet abrasives in Munchuan County are mainly used for grinding picture tubes, oscilloscope tubes, single crystal silicon, grinding lenses, lenses, glasses, grinding flat frosted glass, craft glass cups and bottles, grinding metal printing plates and prongs of inverted glass plates, etc. Xingtai in Hebei, star mine in Ulat Houqi, Inner Mongolia, Danfeng mine in Shaanxi, Tonghua Guanghua mine in Jilin, Zaoyang in Hubei, etc. also produce garnet.



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